Method of injection molding plastic lens

ABSTRACT

A cavity  22  in an injection molding assembly includes a lower mold insert  21  for shaping a lens convex surface and an upper mold insert  20  for shaping a lens concave surface. When the heated injection molding assembly is cooled and a lens is ejected after a molten resin is pressurized by the upper mold insert  20 , the temperature of the lower mold insert  21  is lowered below the temperature of the upper mold insert  20 , which prevents the molded lens from bending at a central portion and enable high transfer precision of insert shapes.

TECHNICAL FIELD

The present invention relates to a method of injection molding a plasticlens and, more particularly, to temperature control of an injectionmolding assembly for making a highly precise lens molded in a cavity.

BACKGROUND ART

An injection molding technology to mold a meniscus-shaped plasticspectacle lens is shown in Japanese Patent Publication No. Hei 5-30608.In this technology, a cavity for molding the lens is formed inside aninjection molding assembly, the cavity containing a pair of cavityforming members for shaping a convex surface and a concave surface ofthe lens disposed vertically opposite with each other. The injectionmolding assembly is heated before filling a molten resin in the cavityand one cavity forming member is moved toward the other cavity formingmember to pressurize the molten resin filled in the cavity.Subsequently, the injection molding assembly is cooled to cool andsolidify the molten resin, and the molded lens is taken out (=eject).

It is shown in Japanese Patent Laid-open No. Hei 6-31785 that aninjection molding assembly is heated by means of a heating fluid such assteam and cooled by means of a cooling fluid such as air, water. Inaddition, after a molten resin is filled in a cavity in the injectionmolding assembly of which the temperature is raised beyond flow haltingtemperature of the molten resin, the temperature of the injectionmolding assembly is lowered below a glass transition point for molding alens by cooling and solidifying the molten resin.

A lens is a precise molded product which requires high moldingprecision. Especially in a meniscus lens used for a spectacle lens, itis important that a convex shape and a concave shape of a pair of cavityforming members for shaping a convex surface and a concave surface ofthe lens are precisely transferred to the lens. However, when a lens tobe molded has a difference in thickness between a central portion and aperipheral portion thereof, a thickness of the central portion beinglarger less than that of the peripheral portion, the lens is easy tobend at the thin central potion portion. When such a disadvantageoccurs, a high-precision lens to which a convex shape and a concaveshape of cavity forming members are accurately transferred is notobtained.

For manufacturing a high-precision lens, it is important to prevent heatdistortion or shrinkage deformation from occurring, which requires thatthe entire molten resin filled in the cavity is uniformly cooled.However, since the amount of the molten resin filled in the cavitycorresponds to the volume of the lens and differs depending on a type ofthe lens, especially lens power, uniform cooling is difficult bycontrolling the temperature uniformly. Thus, temperature control of aninjection molding assembly is desired for molding each lens highlyprecisely irrespective of the above difference.

An object of the present invention is to provide a plastic lensinjection molding method to mold a high-precision lens by means ofproper temperature control of an injection molding assembly.

DISCLOSURE OF THE INVENTION

A method of injection molding a plastic lens according to the presentinvention provides a cavity for molding the lens formed by a pair ofcavity forming members disposed opposite with each other inside aninjection molding assembly for shaping a convex surface and acorresponding concave surface of the lens. The injection moldingassembly is heated before filling a molten resin in the cavity andpressurizing the molten resin. Thereafter, the injection moldingassembly is cooled to cool and solidify the molten resin for molding thelens in the cavity, before ejecting from the cavity. In theaforementioned method of injection molding the plastic lens, thetemperature of the cavity forming member for shaping the lens convexsurface is lowered below the temperature of the cavity forming memberfor shaping the lens concave surface in ejecting the lens.

According to the above injection molding method, when the lens isejected, the temperature of the lens convex surface is lower than thatof the lens concave surface, that is, the lens convex surface is cooledand solidified earlier from the lens concave surface, which prevents thelens from bending at a central portion thereof. Consequently, ahigh-precision lens can be obtained, where shapes of the convex surfaceand the corresponding concave surface of a pair of the cavity formingmembers are precisely transferred.

The aforementioned injection molding method is used for molding ameniscus lens, especially more effective in molding a lens having largerthickness of a peripheral portion than the thickness of a centralportion (a minus lens).

When the minus lens is molded, it is preferable that the difference intemperature between the cavity forming member for shaping the lensconvex surface and the cavity forming member for shaping the lensconcave surface is enlarged in proportion to increase in the power(meaning spherical vertex refractive power and/or cylindrical refractivepower in the present invention) of the lens molded in the cavity.Usually, as the lens power increases, the thickness of a peripheralportion becomes larger than that of a central portion, that is, adifference in thickness is enlarged, which causes the lens to bendeasily at the central portion. However, the central portion of the lensis prevented from being bent even in a minus lens having large thicknessdifference by enlarging the temperature difference between the cavityforming member for shaping the lens convex portion and cavity formingmember for shaping the lens concave portion in proportion to theincrease in the lens power.

Time to cool the injection molding assembly after pressurization of themolten resin is preferably lengthened in proportion to the increase inthe lens power in order to mold each of highly precise lenses havingdifferent power. As the lens power increases, the volume of the lens,that is, the amount of the molten resin filled in cavity increases.Therefore, the whole molten resin in the cavity can be gradually cooleduniformly to a predetermined temperature by lengthening a cooling timein proportion to the increase of the lens power. Consequently, each oflenses with different powers can be molded highly precisely with littleheat distortion, little shrinkage deformation and the like.

In order to lower the temperature of the cavity forming member forshaping the lens convex surface below the temperature of the cavityforming member for shaping the lens concave surface, but thetemperatures of the two cavity forming members may be the same or almostthe same over the majority of the cooling time after pressurization ofthe molten resin. However, in order to securely lower the temperature ofthe lens convex surface below the temperature of the lens concavesurface in ejecting, it is preferable that the cooling time of theinjection molding assembly is controlled while differentiating thetemperature of the cavity forming member for shaping the lens concaveportion and the cavity forming member for shaping the lens convexportion by controlling flow rate of the temperature controlling fluidcirculating in the injection molding assembly for raising and loweringthe temperature of the injection molding assembly, thereby lowering thetemperature of the cavity forming member for shaping the lens convexsurface below the temperature of the cavity forming member for shapingthe lens concave surface.

In the above, a pair of the cavity forming members for shaping theconvex surface and the concave surface of the lens may be opposed witheach other vertically or horizontally. In other words, an injectionmolding machine in which the injection molding assembly is mounted canbe vertically or horizontally structured.

The number of cavities provided in the injection molding assembly isoptional. One or more than one cavity is available.

In the method of injection molding the plastic lens according to thepresent invention, the molten resin is filled in the cavity for moldingthe lens inside the heated injection molding assembly and pressurized.Subsequently, the injection molding assembly is cooled to cool andsolidify the molten resin so as to mold the lens in the cavity beforeejecting the lens from the cavity. The injection molding method ischaracterized in that the time to cool the injection molding assembly islengthened in proportion to increase in the power of the lens molded inthe cavity.

According to the aforementioned injection molding method, since thecooling time of the injection molding assembly is controlled inproportion to change in power, the entire molten resin can be uniformlycooled to the predetermined temperature, thereby manufacturing ahigh-precision lens with little heat distortion, little shrinkagedeformation and the like.

The injection molding method is available for molding, a lens of which athickness of a peripheral portion is smaller than that of a centralportion (a plus lens), and a semi-finished lens as well as theaforementioned minus lens.

Moreover, the injection molding method is applicable not only formolding a meniscus lens but for molding other types of lenses.

When the lens is a spectacle lens, some spectacle lenses have the samelens power, and different astigmatic powers. In such spectacle lenseswith the same power and different astigmatic powers, it is preferablethat the cooling time of the injection molding assembly is lengthened inproportion to increase in the astigmatic power, since the amount and/orthe shape of molten resin filled in the cavity change in accordance withthe astigmatic power even when the surface area of cavities in theinjection molding assembly have no substantial difference.

The injection molding assembly can be heated and cooled by means of anelectric heater, air cooling and the like. However, if the injectionmolding assembly is heated and cooled by means of a temperaturecontrolling fluid of which the temperature is controlled, morespecifically a heating fluid and a cooling fluid, temperature controlcan be performed highly precisely and easily.

When the lens is a spectacle lens, it is preferable that the temperatureof the injection molding assembly is controlled in accordance withtemperature curve, at least two temperature curves being prepared forweak power and strong power for minus lens and at least one for the pluslens. In molding a semi-finished lens of which one surface is laterprocessed, it is preferable that another temperature curve is furtherprepared and the temperature of the injection molding assembly iscontrolled in accordance with the temperature curve. Thus, at leastthree temperature curves of the injection molding assembly, that is, fora weak minus lens, a strong minus lens and a plus lens, or fourtemperature curves can be prepared when a temperature curve for asemi-finished lens is included, so that temperature control can beeasily conducted.

When the minus lens and the plus lens are molded, it is preferable thattime to cool the injection molding assembly after pressurization of themolten resin is set for respective groups divided by lens sphericalpower and lens astigmatic power. Especially in molding the minus lens,the cooling time is preferably set for respective groups divided on thebasis of the sum of lens spherical power and lens astigmatic power.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical sectional view of an injection molding assemblyused for implementing a molding method according to an embodiment of thepresent invention;

FIG. 2 is a sectional view taken along the II—II line FIG. 1;

FIG. 3 is a sectional view taken along the III—III line FIG. 1;

FIG. 4 is a schematic view showing an apparatus for adjusting andcontrolling the temperature of the molding assembly;

FIG. 5 is a fragmentary enlarged view of FIG. 1 in molding a minus lenshaving thicker peripheral portion than a central portion;

FIG. 6 is a perspective view of a whole minus lens;

FIG. 7 is a sectional view taken along the VII—VII line in FIG. 6;

FIG. 8 is a sectional view taken along the VIII—VIII line in FIG. 6;

FIG. 9 is a fragmentary enlarged view of FIG. 1 in molding a plus lenshaving thinner peripheral portion is thinner than a central portion;

FIG. 10 is a perspective view of a hole plus lens;

FIG. 11 is a sectional view taken along the XI—XI line in FIG. 10;

FIG. 12 is a sectional view taken along the XII—XII line in FIG. 10; and

FIGS. 13(A)-13(D) are graphical representation showing changes intemperature of an upper mold insert and a lower mold insert for variouskinds of lenses from pressurization of a molten resin to ejection of amolded product;

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below withreference to the drawings. An injection molding method according to thepresent embodiment is for molding a meniscus lens for glasses. Aninjection molding assembly used for the injection molding method isshown in FIG. 1 to FIG. 3. FIG. 2 and FIG. 3 are sectional views takenalong the II—II line and the III—III line in FIG. 1. The injectionmolding assembly can be formed of optional material such as glass andceramic besides metal. Material for spectacle lenses as molded productsis a thermoplastic resin such as PMMA (polymethyl methyacrylate), and PC(polycarbonate).

A structure of the injection molding assembly is hereunder describedwith reference to FIG. 1 to FIG. 3. The injection molding assembly iscomposed of an upper mold 1 and a lower mold 2. The upper mold 1 is amovable mold which opens and closes vertically in relation to the lowermold 2 as s fixed mold, and a parting line PL extends horizontally. Theupper mold 1 is composed of a mold body 3 on a lower side and a diefitting member 4 in an upper side. The mold body 3 is provided withinsert guides 5, mold plates 6 and 7 and the like. The die fittingmember 4 is provided with an upper member 8 and a lower member 9 and thelike. The lower mold 2 is composed of insert guides 10, mold plates 11and 12, a sprue bush 13 and the like.

As clearly shown in FIG. 2, the mold body 3 of the upper mold 1 ismounted on the die fitting member 4 with a bolt 14. In this mounting,the mold body 3 is mounted being guided to the lower mold 2 by means ofa guide rod 15 to be freely movable within a margin S. The margin S isopened between the mold body 3 and the die fitting member 4. The moldbody 3 is always resiliently biased downward by means of a plate spring16 attached on an outer periphery of the bolt 14.

A clamping cylinder (not shown) is provided above the die fitting member4 which is mounted on the clamping cylinder. By the clamping cylinderthe die fitting member 4 and the mold body 3 vertically move and theupper mold 1 composed of the mold body 3 and the die fitting member 4vertically moves to open and close in relation to the lower mold 2. Thisvertical movement is conducted while an end portion 15A of the guide rod15 in the upper mold 1 is inserted into and pulled out from a guide pipe17 in the lower mold 2. The upper mold 1 and the lower mold 2 arealigned in closing the mold by a positioning pin 18 in the upper mold 1being inserted in a positioning sleeve 19 in the lower mold 2.

A margin setting cylinder (not shown) is provided below the lower mold2. When the mold body 3 in the upper mold 1 abuts on the lower mold 2 bythe clamping cylinder and the die fitting member 4 is in close contactwith the mold body 3, the die fitting member 4 is raised againstclamping force of the clamping cylinder with the margin settingcylinder, thus opening the margin S between the mold body 3 and the diefitting member 4.

An upper mold insert 20 is put into the insert guide 5 mounted on themold body 3 in the upper mold 1 movably in vertical direction. A lowermold insert 21 is put into the insert guide 10 provided in the lowermold 2 so as to be movably in vertical direction. By the aforementionedarrangement, a cavity 22 for molding a spectacle lens is formed. Asshown in FIG. 1, two of the cavities 22 are provided on right and leftsides in the present embodiment. Therefore, the injection moldingassembly is used for molding two spectacle lenses simultaneously.

The upper mold insert 20 and the lower mold insert 21 form the cavity 22with the insert guide 5 and 10, that is, the inserts 20 and 21 arecavity forming members. In the present embodiment, the upper mold insert20 is a cavity forming member for shaping a concave surface of the lensand the lower mold insert 21 is a cavity forming member for shaping aconvex surface of the lens.

Each of the upper mold inserts 20 is attached to a piston rod 24 of ahydraulic cylinder 23 disposed downward through a T-shaped clampingmember 25, the hydraulic cylinder 23 being built in the die fittingmember 4 in the upper mold 1 so as to be slideable vertically. Each ofthe lower mold inserts 21 is attached to a piston rod 27 of a hydrauliccylinder 26 disposed upward through a T-shaped clamping member 28, thehydraulic cylinder being fixed on the lower mold 2. A back insert 29, inwhich the piston rod 24 is inserted to be slideable vertically, is fixedon a lower surface of the hydraulic cylinder 23.

When the upper mold 1 is raised by means of the clamping cylinder andhence the upper mold 1 and the lower mold 2 are parted from the partingline PL, the upper mold insert 20 and the lower mold insert 21 areexposed between the upper mold 1 and the lower mold 2 by advancing thepiston rods 24 and 27. T-shaped slots of the inserts 20 and 21, withwhich the T-shaped clamping members 25 and 28 are engaged, extend to anouter region of the inserts 20 and 21 for opening so that the inserts 20and 21 are, respectively, inserted in and released from the piston rods24 and 27, on which the T-shaped clamping members 25 and 28 are mounted,by engagement and disengagement of the T-shaped clamping members 25 and28 with/from the T-shaped slots. Consequently, various insertscorresponding to spectacle lenses to be molded are exchangeably attachedon the upper mold 1 and the lower mold 2. Meanwhile, when the pistonrods 24 and 27 retract, the upper mod insert 20 and the lower moldinsert 21 abut respectively on the back insert 29 and the mold plate 12in the lower mold 2 to be seated, which makes the inserts 20 and 21clamped.

A pressure receiving member 30 mounted on an upper surface of thehydraulic cylinder 23 is accommodated inside a recessed portion 8A ofthe upper member 8 composing the die fitting member 4 in the upper mold1. As shown in FIG. 2, a pair of guide bars 31 slidably inserted in thelower member 9 of the die fitting member 4 are hung from the pressurereceive member 30. By means of springs 32 attached on outer peripheriesof the guide bars 31, the pressure receiving member 30, the hydrauliccylinder 23, and the back insert 29 are always resiliently biased upwardoppositely to the lower mold 2 and the pressure receiving member 30abuts on an upper surface of the recessed portion 8A formed downward inrelation to the upper member 8 of the die fitting member 4.

A through-hole 33 leading to the recessed portion 8A is formed in theupper member 8 of the mold attaching member 4. An eject pin 34 isinserted in the through-hole 33 to move vertically by an eject cylinder(not shown). The eject pin 34 abuts on the pressure receiving member 30,and with descent of the eject pin 34 by means of the eject cylinder, thepressure receiving member 30, the hydraulic cylinder 23, the back insert29, and the upper mold insert 20 are pressurized to move downward inrelation to the upper mold 1.

As shown in FIG. 1, an eject bar 35 is inserted to be movable verticallyin central parts of the mold body 3 of the upper mold 1 and the lowermember 9 of the die fitting member 4. A pair of guide bars 37,vertically slidably inserted in the lower member 9, are fixedly hungfrom an pressure receiving member 36 mounted on an upper end of theeject bar 35 as shown in FIG. 3. By means of spring 38 attached on outerperipheries of the guide bars 37, a pressure receiving member 36 and theeject bar 35 are always resiliently biased upward. An eject pin 40,which is moved vertically with an eject cylinder (not shown), isinserted in a through-hole 39 formed in the upper member 8 of the diefitting member 4. With the eject pin 40, the pressure receiving member36 and the eject bar 35 are pressurized to move downward.

As shown in FIG. 1, an injection nozzle 41 in an injection moldingmachine is connected to the sprue bush 13. A runner 43 is connected toan upper end of a sprue 42 in the sprue bush 13, the runner 43 extendingto the cavities 22 provided on right and left side.

The whole apparatus for adjusting and controlling the temperature of theinjection molding assembly is shown in FIG. 4. Main lines 52 to 55extend from a temperature controlling fluid feeder 51 controlled by acontroller 50. End portions of the main lines 52 to 55 lead to branchlines 52A, 52B, 53A, 53B, 54A, 54B, 55A, and 55B.

The branch lines in pair for every main line 52 to 55 are disposedcorrespondingly to two cavities 22 provided on both sides shown in FIG.1. In other words, the branch lines 52A and 52B are connected to ringslots 57 formed on upper surfaces of two right and left upper moldinserts 20 through passages 56, the branch lines 53A and 53B areconnected to ring slots 59 formed on lower surfaces of two right andleft lower mold inserts 21 through passages 58, the branch lines 54A and54B are connected to peripheral slots 61 formed on side surfaces of tworight and left upper mold insert guides 5 through passages 60, andbranch lines 55A and 55B are connected to peripheral slots 63 formed onside surfaces of two right and left lower mold insert guides 10 throughpassages 62.

The temperature controlling fluid feeder 51 shown in FIG. 4 circulates aheating fluid and a cooling fluid through the aforementioned main lines,branch lines, passages, ring slots and peripheral slots so as to raiseand lower the temperature of the injection molding assembly, morespecifically, temperature around the cavities 22 inside the injectionmolding assembly. The heating fluid is steam and the cooling fluid isair and water, for example. In the temperature controlling fluid feeder51 has a switching valve to switch over the heating fluid and thecooling fluid, a closing value to supply and suspend the fluids and thelike. The switching valves, the closing valves and the like arecontrolled by means of signal from the controller 50 which controls theheating and cooling time of the injection molding assembly by theheating fluid and the cooling fluid from the temperature controllingfluid feeder 51. Incidentally, a discharge line, for discharging theheating fluid and the cooling fluid supplied to the injection moldingassembly is provided in the injection molding assembly, though not shownin the drawing.

A molding process of a plastic lens for glasses using the injectionmolding assembly is conducted as follows. The upper mold 1 and the lowermold 2 are closed by the clamping cylinder. When the margin S is openbetween the mold body 3 of the upper mold 1 and the die fitting member 4upon operation of the margin setting cylinder and a molten resin isready to be filled in the cavity 22, the injection molding assembly isheated by supplying the heating fluid from the temperature controllingfluid feeder 51 to raise the temperature of the cavity 22 beyond flowhalting temperature of the molten resin. Subsequently, the molten resinis injected from the injection nozzle 41 to be filled in the cavity 22through the sprue 42 and the runner 43, and the nozzle is shutthereafter.

After the filling or during the filling, the die fitting member 4 islowered by the clamping cylinder and the molten resin in the cavity 22is pressurized by the amount corresponding to the margin S by the uppermold insert 20. After the pressurization, the cooling fluid is suppliedto the injection molding assembly from the temperature controlling fluidfeeder 51. Thus, the temperature of the molten resin in cavity 22 islowered to, for example, around 100 degrees centigrade below the glasstransition point for molding spectacle lens by cooling andsolidification of the molten resin.

After the spectacle lens is molded by cooling and solidification of themolten resin in the cavity 22, the upper mold 1 is opened from the lowermold 2 by the clamping cylinder. With the descent of the eject pins 34and 40, a molded product is pushed out by the upper mold insert 20 andthe eject bar 35. Subsequently, the molded product provided with twospectacle lenses ejected from the injection molding assembly asdescribed above is cut to obtain the spectacle lenses molded in thecavities 22.

The upper mold insert 20 and the lower mold insert 21 are exchangeableas stated above. When molding process is conducted using the upper moldinsert 20A and the lower mold insert 21A shown in FIG. 5, a minus lens71 having larger a thickness T2 of a peripheral portion than a thicknessT1 of a central portion is molded as shown in FIGS. 6 and 7. FIG. 8 is asectional view taken in a direction perpendicular to FIG. 7. In FIG. 8,a thickness T3 of a peripheral portion is larger than the thickness T2thus making the minus lens 71 as astigmatic spectacle lens by thedifference between T2 and T3. On the other hand, when molding process isconducted using an upper mold insert 20B and a lower mold insert 21Bshown in FIG. 9, a plus lens 72 having smaller thickness T5 ofperipheral portion than a thickness T4 of a central portion can bemolded as shown in FIGS. 10 and 11. FIG. 12 is a sectional view in aright-angled direction perpendicular to FIG. 11. In FIG. 12, a thicknessT6 of peripheral portion is larger than T5, thus making the plus lens 72as astigmatic spectacle lens by the difference between T5 and T6.

Upper mold inserts and lower mold inserts are respectively preparedcorrespondingly to every lens power (diopter) of minus lenses and pluslenses. Besides, another upper mold insert and a lower mold insert areprovided for molding a semi-finished lens, having one surface alreadyfinished and the other surface to be later finished.

(A) to (D) in FIG. 13 show temperature curve of upper mold inserts andlower mold inserts in molding various kinds of lenses with a diameter of76 millimeter from pressurization of the molten resin to ejection of themolded product. FIG. 13(A) to (D) correspond to each lens shape and/orpower. FIG. 13(A) shows a case of molding the minus lens (weak minuslens) with a power of −2.00, T1 of 1.4 mm and T2 of 4.8 mm, FIG. 13(B)shows a case of molding a minus lens (strong minus lens) with a power of−4.00, T1 of 1.4 mm and T2 of 7.9 mm, FIG. 13(C) shows a case of moldinga plus lens with a power of +2.00, T4 of 4.2 mm and T5 of 1.0 mm, andFIG. 13(D) shows a case of molding a semi-finished lens with a basecurve of convex surface of 300 D, a thickness of a central portion of5.4 mm and a thickness of a peripheral portion of 5.8 mm. These figuresare for explaining basic lens molding patterns (basic lens moldingpatterns of a weak minus lens, a strong minus lens, a semi-finished lensand a plus lens) corresponding to each lens shape and/or power.

When a minus lens is molded as shown in FIG. 13(A) and (B), thetemperature of a lower mold insert (namely, a cavity forming member forshaping a convex surface of the lens) in ejecting a molded product fromthe injection molding assembly is lowered below the temperature of anupper mold insert (namely, a cavity forming member for shaping a concavesurface of the lens). When there is such a difference in temperature asabove between the lower mold insert and the upper mold insert accordingto the difference in lens shape and/or power, since the temperature ofthe convex surface of the ejected lens is low even in a minus lens ofwhich thin central portion is likely to be bent and the temperature ishigh when being ejected, the convex side of the lens solidifies earlierthan the concave side thereof, thereby efficiently preventing thecentral portion from being bent. In other words, a concave shape of thelower mold insert and a convex shape of the upper mold insert areprecisely transferred to a molten resin so that a high-precision lenscan be manufactured.

It is also effective in molding a lens of which a thickness of a centralportion is smaller than that of a peripheral portion that thetemperature of the lower mold insert is lowered below the temperature ofthe upper mold insert when the molded product is ejected as describedabove. Therefore, when a semi-finished lens with a small difference inthickness between a central portion thereof and a peripheral portionthereof shown in (D) in FIG. 13 is molded, the temperature of the lowermold insert is also preferably lowered below that of the upper moldinsert as described in molding the minus lens.

As shown in FIG. 13(A), (B), and (D), the larger the difference inthickness between a central portion and a peripheral portion becomes,the more a difference in temperature between the lower mold insert andthe upper mold insert is enlarged, which securely prevents various kindsof lenses with various differences in thickness between central portionsand peripheral portions from bending at central portions thereof.

As shown in FIG. 13(A), (B), and (D), particularly in FIG. 13(A) and(B), when a lens of which a thickness of a peripheral portion is largerthan the thickness of a central portion is molded, the temperature ofthe lower mold insert is lowered below that of the upper mold insertfrom pressurization of the molten resin to ejection of a molded product.As a result, the temperature of a convex surface of the lens can besecurely lowered below the temperature of a concave surface when themolded product is ejected, thereby improving transfer precision ofshapes of the lower mold insert and the upper mold insert.

Following Table 1 shows the cooling time of the injection moldingassembly from the start of supplying the aforementioned cooling liquidafter pressurizing the molten resin to ejection of a molded product whena minus lens and a plus lens are molded. The cooling time is dividedinto groups according to lens power (diopter) (spherical power) andastigmatic power. Especially in the minus lens, the grouping is based onthe value of the sum of the spherical power and the astigmatic power.The lens spherical power is indicated in a vertical axis and theastigmatic power is indicated in a horizontal axis. A three-digit numberin the Table 1 indicates a cooling time (second).

TABLE 1

As can be seen from FIG. 13(A) and (B), the increase of lens power in aminus lens means that a thickness of the peripheral portion T2 becomesmuch larger than the thickness of the central portion T1. Also, theincrease of lens power in a plus lens means that a thickness of thecentral portion T4 becomes much larger than that of the peripheralportion T5. As shown in Table 1, the more a lens power increases, thelonger a cooling time is made in both the minus lens and the plus lens.When a lens power increases, through surface area of the cavity in theinjection molding assembly does not substantially changes, the volume ofa lens, that is, the amount of the molten resin filled in the cavity 22increases. Accordingly, the cooling time is lengthened in proportion tothe increase of the lens power, thereby gradually cooling the wholemolten resin in the cavity 22 uniformly to a predetermined temperature(the temperature for ejecting the molded product). Consequently, ahigh-precision and high-quality plastic spectacle lens with little heatdistortion, little shrinkage deformation and the like can be obtained.

The cooling time of the injection molding assembly is lengthened inproportion to the increase of the lens power also in molding asemi-finished lens.

As can be seen from a comparison of graphs of FIG. 13(A) and (B), aminus lens with the time from pressurization of molten resin to ejectionof the molded product is made longer for minus lens having greater lenspower in a minus lens with a smaller power even when the minus lenseshave substantially the same surface area in the cavity, therebylengthening a cooling time of the injection molding assembly. When alens having larger thickness of a peripheral portion than the thicknessof a central portion is molded, the temperature of the lower mold insertis lowered below the temperature of the upper mold insert and a coolingtime of the injection molding assembly is lengthened as shown in FIG.13(A) and (B), which prevents the lens from bending at the centralportion so as to enable high insert transfer precision and also preventsheat distortion, thus making a high-quality lens.

As described above, the thickness of peripheral portions T2 and T3 of aminus lens and the thickness of peripheral portions T5 and T6 of a pluslens respectively, show the thickness of two points which are 90 degreesapart in a circumferential direction. The increase of difference betweenT2 and T3, and between T5 and T6 leads to the increase of astigmaticpower. Even in spectacle lenses with the same lens power, the volume oflens becomes larger and more molten resin is filled in the cavity 22when an astigmatic power is made larger.

In the molding lenses with the same lens power, the larger theastigmatic power becomes, the longer the cooling time is made as shownin Table 1. Accordingly, the whole molten resin filled in the cavity 22can be uniformly cooled to a predetermined temperature in the samemanner as molding glass-lenses with different powers, thus making ahigh-precision and high-quality spectacle lens.

In the injection molding assembly according to the embodiment describedabove, the cavity forming member for shaping the lens convex surface isdisposed on the lower mold and the cavity forming member for shaping thelens concave surface is disposed on the upper mold, but the reversedisposition is also available. The present invention can be alsoimplemented by means of an injection molding assembly havinghorizontally-opposed cavity forming members.

According to the aforementioned embodiment, the heating fluid and thecooling fluid are used respectively for heating and cooling theinjection molding assembly. However, the present invention is notlimited to the above arrangement and preadjusted temperature controllingfluid may be used. Alternatively, a heating method by means of anelectric heater and the like and a cooling method by means of forcedair-cooling and the like can also be adopted. The cooling time requiredfor cooling to the predetermined temperature is set in accordance withcooling methods.

According to the present invention, the temperature of the cavityforming member for shaping the lens convex surface is lowered below thetemperature of the cavity forming member for shaping the lens concavesurface, which prevents the molded lens from bending at a centralportion. As a result, the shapes of the cavity forming members arehighly-precisely transferred to the lens so that a high-precision andhigh-quality lens as desired can be obtained.

According to the present invention, the cooling time of the injectionmolding assembly after pressurization of the molten resin is lengthenedfor a lens with a large power requiring a larger amount of molten resinto be filled in the cavity, so that the entire molten resin can beuniformly cooled to the predetermined temperature. Consequently, ahigh-precision spectacle lens with little heat distortion and littleshrinkage deformation and the like can be manufactured.

INDUSTRIAL AVAILABILITY

A method of injection molding a plastic lens according to the presentinvention is applicable for molding a plastic lens for glasses, anoptical plastic lens etc. made of a thermo-plastic resin, and especiallyuseful for molding a meniscus-shaped plastic lens for glasses whichrequire high-precision molding.

What is claimed is:
 1. A method of injection molding a plastic lens,comprising the steps of; providing a cavity for molding the lensincluding a pair of cavity forming members for shaping a convex surfaceand a concave surface of the lens disposed oppositely with each otherinside an injection molding assembly; heating the injection moldingassembly; filling a molten resin in the cavity for molding the lens;pressurizing the molten resin; cooling the injection molding assembly tocool and solidify the molten resin to mold the lens in the cavity; andejecting the lens from the cavity, wherein the temperature of the cavityforming member for shaping the convex surface is lowered below thetemperature of the cavity forming member for shaping the concave surfaceduring the cooling of the injection molding assembly.
 2. The method ofinjection molding the plastic lens according to claim 1, wherein thelens is a meniscus lens having a larger thickness of a peripheralportion than a thickness of a central portion.
 3. The method ofinjection molding the plastic lens according to claim 2, wherein adifference in temperature between the cavity forming member for shapingthe convex surface and the cavity forming member for shaping the concavesurface is enlarged in proportion to an increase of the power of thelens molded in the cavity.
 4. The method of injection molding theplastic lens according to claim 1, wherein the time to cool theinjection molding assembly is lengthened in proportion to an increase ofthe power of the lens molded in the cavity.
 5. The method of injectionmolding the plastic lens according to claim 1, wherein the temperatureof the cavity forming member for shaping the convex surface is loweredbelow the temperature of the cavity forming member for shaping theconcave surface from pressurization of the molten resin to ejection ofthe lens.
 6. The method of injection molding the plastic lens accordingto claim 1, wherein the lens is a spectacle lens, and wherein a coolingtime of the spectacle lens with the same power is lengthened inproportion to an increase in the astigmatic power of the spectacle lens.7. The method of injection molding the plastic lens according to claim1, wherein the injection molding assembly is heated and cooled by meansof a temperature controlling fluid.
 8. The method of injection moldingthe plastic lens according to claim 7, wherein a heating fluid and acooling fluid are used as the temperature controlling fluid.
 9. Themethod of injection molding the plastic lens according to claim 1,wherein the lens is a spectacle lens and wherein the injection moldingassembly has at least two temperature curves relating to molding a weaklens and a strong lens for minus lenses and at least one temperaturecurves for a plus lens, the temperature of the injection moldingassembly being controlled in accordance with the temperature curves. 10.The method of injection molding the plastic lens according to claim 9,wherein another different temperature curve is prepared for asemi-finished lens of which one side is later molded, the semi-finishedlens being molded by controlling temperature of the injection moldingassembly in accordance with the different temperature curve.
 11. Themethod of injection molding the plastic lens according to claim 9,wherein the time to cool the injection molding assembly afterpressurization the molten resin is set by being grouped according tolens spherical power and lens astigmatic power when a minus lens and aplus lens are molded.
 12. The method of injection molding the plasticlens according to claim 11, wherein the time to cool the injectionmolding assembly after pressurizing the molten resin is set by beinggrouped on the basis of the sum of lens spherical power and lensastigmatic power in molding the minus lens.
 13. A method of injectionmolding a plastic lens comprising: molding the lens from a molten resinin a cavity formed by a pair of cavity forming members for shaping thelens inside a heated injection molding assembly; cooling the injectionmolding assembly to cool and solidify the molten resin to mold the lensin the cavity; and ejecting the lens from the cavity, wherein bending ofthe lens at a thinner portion of the lens during manufacture of the lensis substantially prevented by at least one of controlling a temperatureof a first of the pair of cavity forming members to be below atemperature of a second of the pair of cavity forming members during thecooling of the injection molding assembly, and controlling a coolingtime of the injection molding assembly to be longer in proportion to anincrease in the power of the lens molded in the cavity.
 14. The methodaccording to claim 13, wherein said first cavity forming member is forshaping a convex surface of the lens and said second cavity formingmember is for shaping a concave surface of the lens.
 15. The methodaccording to claim 13, wherein said molding comprises: heating theinjection molding assembly; filling a molten resin in the cavity formolding the lens; and pressurizing the molten resin.
 16. The methodaccording to claim 13, wherein said bending is substantially preventedby controlling a temperature of a first of the pair of cavity formingmembers to be below a temperature of a second of the pair of cavityforming members.
 17. The method according to claim 14, wherein saidbending is substantially prevented by controlling a temperature of saidfirst cavity forming member to be below a temperature of said secondcavity forming member.
 18. The method according to claim 13, whereinsaid bending is substantially prevented by controlling a cooling time ofthe injection molding assembly to be longer in proportion to an increasein the power of the lens molded in the cavity.
 19. The method accordingto claim 18, wherein the lens is a spectacle lens, and wherein a coolingtime of the spectacle lens with the same power is longer in proportionto an increase in the astigmatic power of the spectacle lens.
 20. Themethod according to claim 18, wherein the lens is a spectacle lens andwherein the injection molding assembly has at least two temperaturecurves relating to molding a weak lens and a strong lens for minuslenses and at least one temperature curve for a plus lens, thetemperature of the injection molding assembly being controlled inaccordance with the temperature curves.
 21. The method according toclaim 20, wherein another different temperature curve is prepared for asemi-finished lens of which one side is later molded, the semi-finishedlens being molded by controlling temperature of the injection moldingassembly in accordance with the different temperature curve.
 22. Themethod according to claim 20, wherein the time to cool the injectionmolding assembly after pressurizing the molten resin is set by beinggrouped according to lens spherical power and lens astigmatic power whena minus lens and a plus lens are molded.
 23. The method according toclaim 22, wherein the time to cool the injection molding assembly afterpressurizing the molten resin is set by being grouped on the basis ofthe sum of lens spherical power and lens astigmatic power in molding theminus lens.
 24. The method according to claim 13, wherein said coolingis conducted in accordance with at least one temperature curve so as tocool and solidify the molten resin to mold the lens in the cavity. 25.The method according to claim 13, wherein the lens is a minus lenshaving a thickness at a central portion of the lens that is less than athickness at a peripheral portion of the lens.
 26. A method of injectionmolding a plastic lens comprising the steps of: filling a molten resinin a cavity formed by a pair of cavity forming members for shaping aconvex surface and a concave surface of the lens inside a heatedinjection molding assembly; pressurizing the molten resin; cooling theinjection molding assembly to cool and solidify the molten resin to moldthe lens in the cavity; and ejecting the lens from the cavity, whereinsaid cooling comprises at least one of (i) controlling a cooling time ofthe injection molding assembly to be longer in proportion to an increasein the power of the lens molded in the cavity, and (ii) controlling atemperature of the cavity forming member for shaping the convex surfaceto be lower than a temperature of the cavity forming member for shapingthe concave surface, during the cooling of the injection moldingassembly.
 27. The method according to claim 26, wherein said coolingcomprises controlling a temperature of the cavity forming member forshaping the convex surface to be lower than a temperature of the cavityforming member for shaping the concave surface.
 28. The method accordingto claim 26, wherein said cooling comprises controlling a cooling timeof the injection molding assembly to be longer in proportion to anincrease in the power of the lens molded in the cavity.
 29. The methodaccording to claim 28, wherein the lens is a spectacle lens, and whereina cooling time of the spectacle lens with the same power is longer inproportion to an increase in the astigmatic power of the spectacle lens.30. The method according to claim 28, wherein the lens is a spectaclelens and wherein the injection molding assembly has at least twotemperature curves relating to molding a weak lens and a strong lens forminus lenses and at least one temperature curve for a plus lens, thetemperature of the injection molding assembly being controlled inaccordance with the temperature curves.
 31. The method according toclaim 30, wherein another different temperature curve is prepared for asemi-finished lens of which one side is later molded, the semi-finishedlens being molded by controlling temperature of the injection moldingassembly in accordance with the different temperature curve.
 32. Themethod according to claim 30, wherein the time to cool the injectionmolding assembly after pressurizing the molten resin is set by beinggrouped according to lens spherical power and lens astigmatic power whena minus lens and a plus lens are molded.
 33. The method according toclaim 32, wherein the time to cool the injection molding assembly afterpressurizing the molten resin is set by being grouped on the basis ofthe sum of lens spherical power and lens astigmatic power in molding theminus lens.
 34. The method according to claim 26, wherein said coolingis conducted in accordance with at least one temperature curve so as tocool and solidify the molten resin to mold the lens in the cavity. 35.The method according to claim 26, wherein the lens is a minus lenshaving a thickness at a central portion of the lens that is less than athickness at a peripheral portion of the lens.
 36. A method of injectionmolding a plastic lens comprising the steps of: filling a molten resinin a cavity formed by a pair of cavity forming members for shaping aconvex surface and a concave surface of the lens inside a heatedinjection molding assembly; pressurizing the molten resin; cooling theinjection molding assembly to cool and solidify the molten resin to moldthe lens in the cavity; and ejecting the lens from the cavity, whereinat least one of the following conditions is satisfied: (i) said coolingcomprises controlling a temperature of the cavity forming member forshaping the convex surface to be lower than a temperature of the cavityforming member for shaping the concave surface, during the cooling ofthe injection molding assembly, and (ii) the lens is a spectacle lensand the injection molding assembly has at least two temperature curvesrelating to molding a weak lens and a strong lens for minus lenses andat least one temperature curve for a plus lens, and wherein thetemperature of the injection molding assembly is controlled inaccordance with the temperature curves.
 37. The method according toclaim 26, wherein said cooling comprises controlling a cooling time ofthe injection molding assembly to be longer in proportion to an increasein a sum of the spherical power and the astigmatic power of the lensmolded in the cavity.